Long period gratings (LPGs) are known to be useful for attenuating the light guided in the fundamental mode of a single-mode optical fiber. An LPG operates by providing a periodic perturbation that couples light from the fundamental, LP01 mode of the fiber into higher-order modes. The higher-order modes may be core-guided, cladding-guided or radiative. If not inherently radiative, higher-order modes are readily stripped from the fiber by bends, tapers, or other well-known extractive elements.
The resonant wavelength at which the LPG is most effective for modal coupling depends upon the period of the LPG, which can be selected by design. The strength of the coupling is subject to control when, for example, the LPG is induced by periodic microbends which are mechanically created by an adjustable, serrated clamp in contact with the fiber. Because both the resonant wavelength and the coupling strength are subject to control, an LPG is a useful building block for important optical elements such as gain-flattening filters.
Although useful for purposes such as optical attenuation and optical filtering, the LPG is known to suffer certain drawbacks. Specifically, when microbends are used to induce the LPG, the modal coupling induced by the LPG is polarization-dependent. For example, it is often desirable to use an LPG to couple LP01 radiation into the mode (actually a scalar modal approximation) designated LP11. The LP11 mode comprises a triplet of exact modal solutions having different polarizations, denominated TM, HE, and TE. (The HE component, in turn, is a doublet whose individual components have mutually orthogonal polarizations.) Each of the three LP11 components TM, HE, and TE will generally resonate with a given LPG at a slightly different wavelength. At each of these resonances, a locally maximum amount of power may be transferred from the fundamental mode to the higher-order mode. Coupling into the HE component is substantially independent of the state of polarization of the input fundamental-mode radiation. At a certain polarization of the fundamental mode, however, coupling into TM will be mostly suppressed, whereas at the orthogonal polarization, coupling into TE will be mostly suppressed. Because the fundamental polarization fluctuates randomly in many practical fiber-optic transmission systems, the optical attenuation produced by the LPG is found to be unstable in such systems.
Thus, there has been a need for some way to stabilize the behavior of an LPG-based optical attenuator.